This invention relates generally to an improvement in a portable nuclear measuring gauge for measuring the physical characteristics, such as density and moisture, of a test material. This type of gauge is commonly used for measuring moisture and density in test materials such as soils and pavement.
Commercially available gauges of this type have a housing with a vertically adjustable source rod extending therethrough. The lower end of the source rod contains a source of gamma and/or neutron radiation and radiation detectors are located within the housing. The gauge is designed to be operated in either a backscatter mode of measurement or a direct transmission mode. In the backscatter mode, the source rod is in a retracted position generally flush with the lower surface of the housing and radiation is directed from the source downwardly into the underlying test material. Radiation from the source which is backscattered by the underlying test material is detected at the detectors. When operated in the direct transmission mode of measurement, a hole is formed in the test material and the source rod is moved to a downwardly extended position within the hole. Radiation which passes from the source directly through the test material is detected by the detector and the measurement reading is obtained. In order that the proper calibration parameters are used by the gauge during the calculations which determine the measurement reading, it is necessary to know the depth position of the radiation source in the test material. One well known commercial model of gauge (e.g., Troxler Electronic Laboratories 3400-B Series Surface Moisture-Density Gauge) employs a switch which is manually set by the operator to the same value as the depth of the source rod.
Nuclear gauges have also been proposed in which the depth position of the source rod is automatically determined, as is shown for example in U.S. Pat. No. 4,146,796. In the arrangement shown in this patent a series of switches is provided along the length of the source rod and the switches are selectively actuated as the source rod is moved into different preselected depth positions.